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Sender Policy Framework: Authorizing Use of Domains in E-MAIL
4615 Meredeth #9Lincoln NebraskaNE68506United States of Americawayne@midwestcs.comhttp://www.midwestcs.com/spf/Network Working Group
E-mail on the Internet can be forged in a number of ways. In
particular, existing protocols place no restriction in what a sending
host can use as the reverse-path of a message. This document
describes a protocol whereby a domain can explicitly authorize the
hosts that are allowed to use its domain name in a reverse-path, and
a way for receiving hosts to check such authorization.
The current e-mail infrastructure has the property that any host
injecting mail into the mail system can identify itself as any domain
name it wants. Hosts can do this at a variety of levels: in
particular, the session, the envelope, and the mail headers. While
this feature is desirable in some circumstances, it is a major
obstacle to reducing end-user unwanted e-mail (or "spam").
Furthermore, many domain name holders are understandably concerned
about the ease with which other entities may make use of their domain
names, often with intent to impersonate.
This document defines a protocol by which domain owners may
authorize hosts to use their domain name in the "MAIL FROM" or
"HELO" identity. Compliant domain holders publish SPF records
about which hosts are permitted to use their names, and
compliant mail receivers use the published SPF records to test
the authorization of hosts using a given "HELO" or "MAIL FROM"
identity during a mail transaction.
An additional benefit to mail receivers is that when the use of an
identity is verified, then local policy decisions about the mail can
be made on the basis of the domain, rather than the host's IP
address. This is advantageous because reputation of domain names is
likely to be more accurate than reputation of host IP addresses.
Furthermore, if a claimed identity fails verification, then local
policy can take stronger action against such e-mail, such as
rejecting it.
SPF has been in development since the Summer of 2003, and
has seen deployment beyond the developers beginning in
December, 2003. The design of SPF slowly evolved until the
spring of 2004 and has since stabilized. There have been
quite a number of forms of SPF, some written up as
documents, some submitted as Internet Drafts, and many
discussed and debated in development forums.
The goal of this document is to clearly document the
protocol defined by earlier drafts specifications of SPF as
used in existing implementations. This conception of SPF is
sometimes called "SPF Classic". It is understood that
particular implementations and deployments may differ from,
and build upon, this work. It is hoped that we have
nonetheless captured the common understanding of SPF version
1.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" in this document are to be interpreted as
described in .
This document is concerned with a portion of a mail message
commonly called "envelope sender", "return path", "reverse
path", "bounce address", "2821 FROM", or "MAIL FROM". Since
these terms are either not well defined, or often used
casually, this document defines the "MAIL FROM" identity in
. Note that other terms, that
may superficially look like the common terms, such as
"reverse-path" or "Return-Path" are used only with the
defined meanings from normative documents.
The "HELO" identity derives from either the SMTP HELO or
EHLO command (see .) These commands
supply the SMTP client (sender) for the STMP session. Note
that requirements for the domain presented in the EHLO and
HELO commands are not strict, and software must be prepared
for the "HELO" identity to be malformed.
SPF clients MAY check the "HELO" identity by calling the
check_host() function () with the
"HELO" identity as the <sender>. If the HELO test
returns a "fail", the overall result for the SMTP session is
"fail", and there is no need to test the "MAIL FROM" identity.
The "MAIL FROM" identity derives from the SMTP MAIL command
(see .) This command supplies the
"reverse-path" for a message, which generally consists of
the sender mailbox, and is the mailbox to which notification
messages are sent if there are problems delivering the
message.
allows the reverse-path to be null
(see Section 4.5.5.) In this case, there is no explicit
sender mailbox, and such a message can be assumed to be a
notification message from the mail system itself. When the
reverse-path is null, this document defines the "MAIL FROM"
identity to be the mailbox composed of the localpart
"postmaster" and the "HELO" identity
SPF clients MUST check the "MAIL FROM" identity unless HELO
testing produced a "fail". SPF clients check the "MAIL
FROM" identity by calling the check_host() function with the
"MAIL FROM" identity as the <sender>.
An SPF compliant domain MUST publish a valid SPF record as
described in . This record
authorizes the use of the domain name in the "HELO" and/or
"MAIL FROM" identity, by some sending MTAs, and not by
others.
It is RECOMMENDED that domains publish SPF records that end
in "-all", or redirect to other records that do, so that a
definitive determination of authorization can be made.
Domain holders may publish SPF records that explicitly
authorize no hosts for domain names that shouldn't be used
in sender mailboxes.
A mail receiver can perform an SPF compliant check for each
mail message it receives. This check tests the
authorization of a client host to inject mail with a given
"MAIL FROM" identity. This check MAY also be applied to the
"HELO" identity. Typically, such checks are done by a
receiving MTA, but can be performed elsewhere in the mail
processing chain so long as the required information is
available.
It is expected that mail receivers will use the SPF check as
part of a larger set of tests on incoming mail. The results
of other tests may influence whether or not a particular SPF
check is performed. For example, finding the sending host
on a local white list may cause all other tests to be
skipped and all mail from that host to be accepted.
When a mail receiver decides to perform an SPF check, it
MUST implement and evaluate the check_host() function () correctly. While the test as a whole
is optional, once it has been decided to perform a test it
must be performed as specified so that the correct semantics
are preserved between publisher and receiver.
To make the test, the mail receiver MUST evaluate the
check_host() with the arguments set as follows:
- the IP address of the SMTP client
that is injecting the mail, either IPv4 or IPv6.
- the domain portion of the
"MAIL FROM" or "HELO" identity.
- the "MAIL FROM" or "HELO"
identity.
Note that the <domain> argument may not be a well
formed domain name. For example, if the reverse-path was
null, then the EHLO or HELO domain is used. In a valid SMTP
session, this can be an address literal or entirely
malformed. In these cases, check_host() is defined in to return a "None" result.
Care must be taken to correctly extract the <domain>
from the <sender> as many MTAs will still accept such
things as source routes (see
appendix C), the percent hack (see )
and bang paths (see ). Spammers
often use of such archaic features to try and trick MTAs
into being open relays.
Software SHOULD perform this authorization check during the
processing of the SMTP transaction that injects the mail.
This allows errors to be returned directly to the injecting
server by way of SMTP replies. Software can perform the
check as early as the MAIL command, though it may be easier
to delay the check to some later stage of the transaction.
Software can perform the authorization after the
corresponding SMTP transaction has completed. There are two
problems with this approach: 1) It may be difficult to
accurately extract all the required information such as
client IP address and HELO domain name. 2) If the
authorization fails, then generating a non-delivery
notification to the alleged sender is problematic as such an
action would go against the explicit wishes of the alleged
sender.
The check_host() function returns one of seven results.
This section describes how software that performs the
authorization must interpret the results. If the check is
being performed during the SMTP mail transaction, it also
describes how to respond.
A result of None means that no records were published by
the domain. The checking software cannot ascertain if the
client host is authorized or not.
The domain owner has explicitly stated that doesn't know
whether the IP is authorized or not. A Neutral result
MUST be treated exactly like the None result.
A Pass result means that the client is authorized to
inject mail with the given identity. Further
policy checks, such as reputation, or black and/or white
listing, can now proceed with confidence in the identity.
A Fail result is an explicit statement that the client is
not authorized to use the domain in the given
identity. The checking software can choose to mark the
mail based on this, or to reject the mail outright.
If the checking software chooses to reject the mail during
the SMTP transaction, then it MUST use a 550 reply code
with an appropriate message. The check_host() function
may return either a default explanation string, or one
from the domain that published the SPF records (see ). If the information doesn't originate
with the checking software, it should be made clear that
text is not trusted. For example:
550-SPF MAIL FROM check failed:
550-The domain example.com explains:
550 Please see http://www.example.com/mailpolicy.html
A SoftFail result should be treated as somewhere between a
Fail and a Neutral. This value is used by domains as an
intermediate state during roll-out of publishing records.
The domain believes the host isn't authorized but isn't
willing to make that strong of a statement. Receiving
software SHOULD NOT reject the message based on this
result, but MAY subject the message to closer scrutiny.
Because the domain has discouraged any legitimate use of
this IP address, receivers MAY try to inform either the
sender or the recipient of the e-mail. For example, the MUA
could highlight the SoftFail status for the receiver, or the
MTA could issue a tempfail SMTP code (4xx) with a note the
first time the message was received, but accept it the
second time.
A TempError result means that the receiving server
encountered a transient error when performing the check.
Checking software can choose to accept or temporarily
reject the message. If the message is rejected during the
SMTP transaction for this reason, the software MUST use a
450 reply code.
A PermError result means that the domain's published
records couldn't be correctly interpreted. Checking
software SHOULD reject the message. If rejecting during
SMTP transaction time, a 550 reply MUST be used.
An SPF record declares which hosts are, and are not,
authorized to use a domain name for the "HELO" or "MAIL FROM"
identity. Loosely, the record partitions all hosts into
permitted and not-permitted sets. (Though some hosts might
fall into other categories.)
The SPF record is a single string of text. An example record is:
v=spf1 +mx a:colo.example.com/28 -all
This record has a version of "v=spf1" and three directives:
"+mx", "a:colo.example.com/28" (the + is implied), and "-all".
Domain owners wishing to be SPF compliant must publish SPF
records for the hosts that are used in both the MAIL FROM
and HELO identities. The SPF records are placed in the DNS
tree at the host name it partains to, not a subdomain under
it, such as is done with SRV records. This is the same
whether TXT RRs or SPF RRs are used.
The example above in might be
published easily via this lines in a domain zone file:
example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"smtp-out.example.com. IN TXT "v=spf1 a -all"
When published via TXT records, there may be other TXT
records for other purposes published there and this may
cause problems with size limits (see .)
An SPF record published at the zone cut for the domain will
be used as a default for all other domains and subdomains
within the zone. See for details.
Domain owners SHOULD publish SPF records for hosts used for
the HELO and MAIL FROM identities instead of using the zone
cut default because the fallback requires additional DNS
lookups. The zone cut default does reduce the need to
publish SPF records for non-email related hosts, such as
www.example.com.
This document defines a new DNS Resource Record (RR) of
type SPF, type code to be determined. The format of this
type is identical to the TXT RR .
For either type, the character content of the record is
encoded as US-ASCII.
It is recognized that the current practice (using a TXT
record) is not optimal, but it is necessary because
there are a number of DNS server and resolver
implementations in common use that cannot handle the new
RR type. The two record type scheme provides a forward
path to the better solution of using a RR type reserved
for this purpose.
An SPF compliant domain name SHOULD have SPF records of
both RR types. A compliant domain name MUST have a record
of at least one type. If a domain has records of both
types, they MUST have identical content. For example,
instead of just publishing one record as in above, it is better to publish:
example.com. IN TXT "v=spf1 +mx a:colo.example.com/28 -all"
example.com. IN SPF "v=spf1 +mx a:colo.example.com/28 -all"
An SPF compliant check SHOULD lookup both types. Lookups
can be performed serially or in parallel. If both types of
records are obtained for a domain, the SPF type MUST be
used.
Example RRs in this document are shown with the TXT record
type, however they could also be published with both RR types.
A domain name MUST NOT have multiple records that would
cause an authorization check to select more than one
record. See for the selection
rules.
A text DNS record (either TXT and SPF RR types) can be
composed of more than one string. If a published record
contains multiple strings, then the record MUST be treated
as if those strings are concatenated together without
adding spaces. For example:
IN TXT "v=spf1 .... first" "second string..."MUST be treated as equivalent toIN TXT "v=spf1 .... firstsecond string..."
SPF or TXT records containing multiple strings are useful
in order to construct longer records which would otherwise
exceed the maximum length of a string within a TXT or SPF
RR record.
The published SPF record for a given domain name SHOULD
remain small enough that the results of a query for it
will fit within 512 octets. This will keep even older DNS
implementations from falling over to TCP. Since the
answer size is dependent on many things outside the scope
of this document, it is only possible to give this
guideline: If the combined length of the DNS name and the
text of all the records of a given type (TXT or SPF) is
under 480 characters, then DNS answers should fit in UDP
packets. Note that when computing the sizes for queries
of the TXT format, one must take into account any other
TXT records published at the domain name. Records that
are too long to fit in a single UDP packet MAY be
silently ignored.
Use of wildcard records for publishing is not
recommended. Care must be taken if wildcard records are
used. If a domain publishes wildcard MX records, it may want
to publish wildcard declarations, subject to the same
requirements and problems. In particular, the declaration
must be repeated for any host that has any RR records at
all, and for subdomains thereof. For example, the example
given in , Section 4.3.3, could be
extended with:
X.COM MX 10 A.X.COM
X.COM TXT "v=spf1 a:A.X.COM -all"
*.X.COM MX 10 A.X.COM
*.X.COM TXT "v=spf1 a:A.X.COM -all"
A.X.COM A 1.2.3.4
A.X.COM MX 10 A.X.COM
A.X.COM TXT "v=spf1 a:A.X.COM -all"
*.A.X.COM MX 10 A.X.COM
*.A.X.COM TXT "v=spf1 a:A.X.COM -all"
Notice that SPF records must be repeated twice for every
name within the domain: Once for the name, and once with a
wildcard to cover the tree under the name.
Use of wildcards is discouraged in general as they cause every
name under the domain to exist and queries against arbitrary names
will never return RCODE 3 (Name Error).
The check_host() function fetches SPF records, parses them,
and interprets them to evaluate if a particular host is or is
not permitted to send mail with a given
identity. Mail receivers that perform this check MUST
correctly evaluate the check_host() function as described here.
Implementations MAY use a different algorithm than the canonical
algorithm defined here, so long as the results are the same.
The function check_host() takes these arguments: - the IP address of the SMTP client
that is injecting the mail, either IPv4 or IPv6.
- the domain portion of the
"MAIL FROM" or "HELO" identity.
- the "MAIL FROM" or "HELO"
identity.
The domain portion of <sender> will usually be the
same as the <domain> argument when check_host() is
initially evaluated. However, it will generally not be true for
recursive evaluations (see
below).
Actual implementations of the check_host() function will
likely need additional arguments.
The function check_host() can result in one of seven results
described in . Based on the
result, the action to be taken is determined by the local
policies of the receiver.
If the <domain> is malformed or is not a fully
qualified domain name, check_host() immediately returns the
result "None".
If the <sender> has no localpart, substitute the
string "postmaster" for the localpart.
In accordance with how the records are published, see above, a DNS query needs to be made
for the <domain> name, querying for either RR type
TXT, SPF or both.
If the DNS lookup returns a server failure (RCODE 2), or
other error (RCODE other than 0 or 3), or the query times
out, check_host() exits immediately with the result
"TempError"
Records begin with a version section:
record = version terms *SP
version = "v=spf1"
Starting with the set of records that were returned by the lookup,
record selection proceeds in two steps:
If any records of type SPF are in the set, then all
records of type TXT are discarded.
Records that do not begin with a version section of
exactly "v=spf1" are discarded. Note that the version
section is terminated either by a SP character or the
end of the record. A record with a version section of
"v=spf10" does not match and must be discarded.
After the above steps, there should be exactly one record
remaining and evaluation can proceed. If there are two or
more records remaining, then check_host() exits immediately
with the result of "PermError".
If no matching records are returned for the <domain;>,
the SPF client MUST find the Zone Cut as defined in section 6 and repeat the above steps.
The <domain>'s zone origin is then searched for SPF
records. If an SPF record is found at the zone origin, the
<domain> is set to the zone origin as if a "redirect"
modifier was executed.
If no matching records are returned for either search, an SPF
client MUST assume that the domain makes no SPF declarations. SPF
processing MUST abort and return "None".
After one SPF record has been selected, the check_host()
function parses and interprets it to find a result for the
current test. If there are any syntax errors, check_host()
returns immediately with the result "PermError".
Implementations MAY choose to parse the entire record first
and return "PermError" if the record is not syntactically
well formed. However, in all cases, any syntax errors
anywhere in the record MUST be detected.
There are two types of terms: mechanisms and modifiers. A
record contains an ordered list of these mechanisms and
modifiers:
terms = *( 1*SP ( directive / modifier ) )
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists )
modifier = redirect / explanation / unknown-modifier
Most mechanisms allow a ":" or "/" character after the name.
Modifiers always contain an equals ('=') character immediately
after the name, and before any ":" or "/" characters that may be
part of the macro-string.
Terms that do not contain any of "=", ":" or "/" are mechanisms.
As per the definition of the ABNF notation in , mechanism and modifier names are
case-insensitive.
Each mechanism is considered in turn from left to
right. If there are no more mechanisms, the result is
specified in .
When a mechanism is evaluated, one of three things can
happen: it can match, it can not match, or it can throw an
exception.
If it matches, processing ends and the prefix value is
returned as the result of that record. If it does not
match, processing continues with the next mechanism. If
it throws an exception, mechanism processing ends and the
exception value is returned.
The possible prefixes, and the results they return are:
PassFailSoftFailNeutral The prefix is optional and defaults to "+".
When a mechanism matches, and the prefix is "-" so that a
"Fail" result is returned and the explanation string is
computed as described in .
Specific mechanisms are described in .
Modifiers are not mechanisms: they do not return match or
not-match. Instead they provide additional information.
While modifiers do not directly effect the evaluation of
the record, the "redirect" modifier has an effect after
all the mechanisms have been evaluated.
If none of the mechanisms match and there is no "redirect"
modifier, then the check_host() returns a result of
"Neutral". If there is a "redirect" modifier, check_host()
proceeds as defined in .
Note that records SHOULD always either use a "redirect" modifier or an
"all" mechanism to explicitly terminate processing.
For example: v=spf1 +mx -all or v=spf1 +mx redirect=_spf.example.com
Several of these mechanisms and modifiers have a
<domain-spec> section. The <domain-spec>
string is macro expanded (see ). The resulting
string is the common presentation form of a fully qualified DNS name:
A series of labels separated by periods. This domain is called the
<target-name> in the rest of this document.
Note: The result of the macro expansion is not subject to
any further escaping. Hence, this facility cannot produce
all characters that are legal in a DNS label (e.g. the
control characters). However, this facility is powerful
enough to express legal host names, and common utility
labels (such as "_spf") that are used in DNS.
For several mechanisms, the <domain-spec> is optional. If it is
not provided, the <domain> is used as the
<target-name>.
This section defines two types of mechanisms.
Basic mechanisms contribute to the language framework. They do not
specify a particular type of authorization scheme.
allinclude
Designated sender mechanisms are used to designate a set of
<ip> addresses as being permitted or not to use the
<domain> for sending mail.
amxptrip4ip6exists
The following conventions apply to all mechanisms that perform a
comparison between <ip> and an IP address at any point:
If no CIDR-length is given in the directive, then <ip>
and the IP address are compared for equality.
If a CIDR-length is specified, then only the specified number of
high-order bits of <ip> and the IP address are compared
for equality.
When any mechanism fetches host addresses to compare with
<ip>, when <ip> is an IPv4 address, A records are
fetched, when <ip> is an IPv6 address, AAAA records are
fetched. Even if the SMTP connection is via IPv6, an
IPv4-mapped IPv6 IP address (see
section 2.5.5) MUST still be considered an IPv4 address.
Several mechanisms rely on information fetched from DNS. For
these DNS queries, except where noted, if the DNS server
returns an error (RCODE other than 0 or 3) or the query times
out, the mechanism throws the exception "TempError". If the
server returns "domain does not exist" (RCODE 3), then
evaluation of the mechanism continues as if the server
returned no error (RCODE 0) and zero answer records.
all = "all"
The "all" mechanism is a test that always matches. It is used as the
rightmost mechanism in a record to provide an explicit default.
For example: v=spf1 a mx -all
Mechanisms after "all" will never be tested. Any "redirect"
modifier () has no effect when
there is an "all" mechanism.
include = "include" ":" domain-spec
The "include" mechanism triggers a recursive evaluation of
check_host(). The domain-spec is expanded as per . Then check_host() is evaluated with the
resulting string as the <domain>. The <ip> and
<sender> arguments remain the same as in the current
evaluation of check_host().
In hind sight, the name "include" was poorly chosen. Only
the evaluated results of the referenced SPF record is used,
rather than acting as if the referenced SPF record was
literally included in the first. Better names for this
mechanism would have been something like "on-pass" or
"if-pass".
The "include" mechanism makes it possible for one domain to
designate multiple administratively independent domains.
For example, a vanity domain "example.net" might send mail
using the servers of administratively independent domains
example.com and example.org.
Example.net could say "v=spf1 include:example.com include:example.org -all".
That would direct check_host() to, in effect, check the
records of example.com and example.org for a "pass"
result. Only if the host were not permitted for either of
those domains would the result be "Fail".
Whether this mechanism matches or not, or throws an error
depends on the result of the recursive evaluation of
check_host():
A recursive check_host() result of:Causes the "include" mechanism to:PassmatchFailnot matchSoftFailnot matchNeutralnot matchTempErrorthrow TempErrorPermErrorthrow PermErrorNonethrow PermError
The "include" mechanism is intended for crossing
administrative boundaries. While it is possible to use
includes to consolidate multiple domains that share the same
set of designated hosts, domains are encouraged to use
redirects where possible, and to minimize the number of
includes within a single administrative domain. For example,
if example.com and example.org were managed by the same
entity, and if the permitted set of hosts for both domains
were "mx:example.com", it would be possible for example.org
to specify "include:example.com", but it would be preferable
to specify "redirect=example.com" or even "mx:example.com".
This mechanism matches if <ip> is one of the
<target-name>'s IP addresses.
A = "a" [ ":" domain-spec ] [ dual-cidr-length ]
An address lookup is done on the <target-name>. The
<ip> is compared to the returned address(es). If any
address matches, the mechanism matches.
This mechanism matches if <ip> is one of the MX hosts
for a domain name.
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
check_host() first performs an MX lookup on the
<target-name>. Then it performs an address lookup on
each MX name returned. The <ip> is compared to each
returned IP address. To prevent Denial-of-Service attacks, a
limit of 10 MX names MUST be enforced (see ). If any address matches, the
mechanism matches.
Note regarding implicit MXes: If the <target-name> has
no MX records, check_host() MUST NOT pretend the target is its single
MX, and MUST NOT default to an A lookup on the
<target-name> directly. This behavior breaks with the
legacy "implicit MX" rule. See Section 5. If
such behavior is desired, the publisher should specify an "a"
directive.
This mechanism tests if the DNS reverse mapping for
<ip> exists and validly points to a domain name within a
particular domain.
PTR = "ptr" [ ":" domain-spec ]
First the <ip>'s name is looked up using this
procedure: perform a DNS reverse-mapping for <ip>,
looking up the corresponding PTR record in "in-addr.arpa."
if the address is an IPv4 one and "ip6.arpa." if it is an
IPv6 address. For each record returned, validate the domain
name by looking up its IP address. To prevent
Denial-of-Service attacks, a limit of 10 PTR names MUST be
enforced (see ). If <ip>
is among the returned IP addresses, then that domain name is
validated. In pseudocode:
sending-domain_names := ptr_lookup(sending-host_IP);
if more than 10 sending-domain_names are found, use at most 10.
for each name in (sending-domain_names) {
IP_addresses := a_lookup(name);
if the sending-domain_IP is one of the IP_addresses {
validated-sending-domain_names += name;
}
}
Check all validated domain names to see if they end in the
<target-name> domain. If any do, this mechanism
matches. If no validated domain name can be found, or if none
of the validated domain names end in the <target-name>,
this mechanism fails to match. If a DNS error occurs while
doing the PTR RR lookup, then this mechanism fails to match.
If a DNS error occurs while doing an A RR lookup, then that
domain name is skipped and the search continues.
Pseudocode:
for each name in (validated-sending-domain_names) {
if name ends in <domain-spec>, return match.
if name is <domain-spec>, return match.
}
return no-match.
This mechanism matches if the <target-name> is either an
ancestor of a validated domain name, or if the
<target-name> and a validated domain name are the same.
For example: "mail.example.com" is within the domain "example.com",
but "mail.bad-example.com" is not.
Note: Use of this mechanism is discouraged because it is
slow, is not as reliable as other mechanisms in cases of DNS
errors and it places a large burden on the arpa name
servers. If used, proper PTR records must be in place for
the domain's hosts and the "ptr" mechanism should be one of
the last mechanisms checked.
These mechanisms test if <ip> is contained within a
given IP network.
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = as per conventional dotted quad notation,
e.g. 192.0.2.0
ip6-network = as per [RFC 3513], section 2.2,
e.g. 2001:DB8::CD30
The <ip> is compared to the given network. If
CIDR-length high-order bits match, the mechanism matches.
If ip4-cidr-length is omitted it is taken to be "/32". If
ip6-cidr-length is omitted it is taken to be "/128". It is
not permitted to omit parts of the IP address instead of
using CIDR notations. That is, use 10.23.45.0/24 instead of
10.23.45.
This mechanism is used to construct an arbitrary domain name
that is used for a DNS A record query. It allows for
complicated schemes involving arbitrary parts of the mail
envelope to determine what is permitted.
exists = "exists" ":" domain-spec
The domain-spec is expanded as per . The resulting domain name is used for a
DNS A RR lookup. If any A record is returned, this mechanism
matches. The lookup type is 'A' even when the connection
type is IPv6.
Domains can use this mechanism to specify arbitrarily complex
queries. For example, suppose example.com publishes the record:
v=spf1 exists:%{ir}.%{l1r+-}._spf.%{d} -all
The <target-name> might expand to
"1.2.0.192.someuser._spf.example.com". This makes fine-grained
decisions possible at the level of the user and client IP address.
This mechanism enables queries that mimic the style of tests that
existing DNSBL lists use.
Modifiers are name/value pairs that provide additional
information. Modifiers always have an "=" separating the
name and the value.
The modifiers defined in this document ("redirect" and
"exp") MAY appear anywhere in the record, but SHOULD
appear at the end, after all mechanisms. Ordering of
these two modifiers does not matter. These modifiers MUST NOT
appear in a record more than once each. If they do, then
check_host() exits with a result of "PermError".
Unrecognized modifiers SHOULD be ignored no matter where in
a record, nor how often. This allows implementations of
this document to handle records with modifiers that are
defined in other specifications.
If all mechanisms fail to match, and a "redirect" modifier is
present, then processing proceeds as follows.
redirect = "redirect" "=" domain-spec
The domain-spec portion of the redirect section is expanded as per
the macro rules in . Then check_host() is
evaluated with the resulting string as the <domain>. The
<ip> and <sender>
arguments remain the same as current evaluation of check_host().
The result of this new evaluation of check_host() is then considered
the result of the current evaluation.
Note that the newly queried domain may itself specify redirect
processing.
This facility is intended for use by organizations that wish to
apply the same record to multiple domains. For example:
la.example.com. TXT "v=spf1 redirect=_spf.example.com"
ny.example.com. TXT "v=spf1 redirect=_spf.example.com"
sf.example.com. TXT "v=spf1 redirect=_spf.example.com"
_spf.example.com. TXT "v=spf1 mx:example.com -all"
In this example, mail from any of the three domains is described by
the same record. This can be an administrative advantage.
Note: In general, the domain "A" cannot reliably use a
redirect to another domain "B" not under the same
administrative control. Since the <sender> stays the
same, there is no guarantee that the record at domain "B"
will correctly work for addresses in domain "A", especially
if domain "B" uses mechanisms involving localparts. An
"include" directive may be more appropriate.
For clarity it is RECOMMENDED that any "redirect" modifier appear as
the very last term in a record.
explanation = "exp" "=" domain-spec
If check_host() results in a "Fail" due to a mechanism match
(such as "-all"), and the "exp" modifier is present, then
the explanation string returned is computed as described
below. If no "exp" modifier is present, then either a
default explanation string or an empty explanation string
may be returned.
The <domain-spec> is macro expanded (see ) and becomes the <target-name>. The
DNS TXT record for the <target-name> is fetched.
If <domain-spec> is empty, or there are any processing
errors (any RCODE other than 0), or if no records are
returned, or if more than one record is returned, then
proceed as if no exp modifier was given.
The fetched TXT record's strings are concatenated with no
spaces, and then treated as an <explain-string>
which is macro-expanded. This final result is the
explanation string.
Software evaluating check_host() can use this string when to
communicate information from the publishing domain in the
form of a short message or URL. Software should make it
clear that the explanation string comes from a third
party. For example, it can prepend the macro string "%{o}
explains: " to the explanation.
Implementations MAY limit the length of the resulting
explanation string to allow for other protocol constraints
and/or reasonable processing limits. The SPF client SHOULD
make it clear when an explanation string is coming from a
third party, such as shown in .
Suppose example.com has this record v=spf1 mx -all exp=explain._spf.%{d}
Here are some examples of possible explanation TXT records at
explain._spf.example.com:
Example.com mail should only be sent by its own servers. a simple, constant message %{i} is not one of %{d}'s designated mail servers. a message with a little more info,
including the IP address that failed the check
See http://%{d}/why.html?s=%{S}&i=%{I} a complicated example that
constructs a URL with the arguments to check_host()
so that a web page can be generated with detailed,
custom instructions
Note: During recursion into an "include" mechanism, exp=
modifiers do not propagate out. In contrast, during
execution of a "redirect" modifier, the explanation string
from the target of the redirect is used.
During processing, an evaluation of check_host() may require
additional evaluations of check_host() due to the "include"
mechanism and/or the "redirect" modifier.
In order to prevent Denial-of-Service attacks, the total
number of DNS lookups must be limited. The subject of a
Denial-of-Service attack can be either the SPF client
directly, the domain owner of the claimed sender, or some
third party domain that is referenced in the SPF record.
Of these, the case of a third party referenced in the SPF
record is the easiest for a DDoS attack to effectively
exploit. For example, a malicious person could create an
SPF record with many references to a victim domain, send
many e-mails to different SPF clients and the SPF clients
would create a DoS attack. In effect, the SPF clients are
being used to amplify the attacker's bandwidth by using
fewer bytes in the SMTP session than is generated by the
DNS queries. Using SPF clients also allows the attacker to
hide the true source of the attack.
As a result, limits that may seem reasonable for an
individual mail server can still allow an unreasonable
amount of bandwidth amplification. Therefore the processing
limits need to be quite small.
SPF implementations MUST limit the number of mechanisms that
do DNS lookups to at most 10. For example, the "mx" and
"include" mechanisms requires DNS lookups, so will count
against this limit, while the "ip4" mechanism does not
require any DNS lookups.
When evaluating the "mx" mechanism, there MUST be a limit of no
more than 10 MXes looked up and checked for matching IP addresses.
When evaluating the "ptr" mechanism or the %{p} macro, there
MUST be a limit of at most 10 PTR DNS records looked up and
checked for a validated domain name.
SPF implementation SHOULD limit the total amount of data
obtained from the DNS queries. For example, when DNS over
TCP or EDNS0 are available, there may need to be an explicit
limit to how much data will be accepted to prevent excessive
bandwidth usage or memory usage.
Implementations must be prepared to handle records that are
set up incorrectly or maliciously.
MTAs or other processors MAY also impose a limit on the
maximum amount of elapsed time to evaluate
check_host(). Such a limit SHOULD allow at least 20
seconds. If such a limit is exceeded, the result of
authentication SHOULD be "TempError".
Domains publishing records SHOULD try to keep the number of
"include" mechanisms and chained "redirect" modifiers to a
minimum. Domains SHOULD also try to minimize the amount of
other DNS information needed to evaluate a record. This can
be done by choosing directives that require less DNS
information and placing lower cost mechanisms earlier in the
SPF record.
For example, consider a domain set up as:
example.com. IN MX 10 mx.example.com.
mx.example.com. IN A 192.0.2.1
a.example.com. IN TXT "v=spf1 mx:example.com -all"
b.example.com. IN TXT "v=spf1 a:mx.example.com -all"
c.example.com. IN TXT "v=spf1 ip4:192.0.2.1 -all"
Evaluating check_host() for the domain "a.example.com"
requires the MX records for "example.com", and then the A
records for the listed hosts. Evaluating for
"b.example.com" only requires the A records. Evaluating for
"c.example.com" requires none.
However, there may be administrative considerations: Using
"a" over "ip4" allows hosts to be renumbered easily. Using
"mx" over "a" allows the set of mail hosts to be changed
easily.
It is RECOMMENDED that SMTP receivers record the result of
SPF processing in the message headers. If an SMTP receiver
chooses to do so, it MUST use the "Received-SPF" header
defined here. This information is intended for the
recipient. (Information intended for the sender described
in , Explanation.)
The header SHOULD be prepended to existing headers, above
the Received: header that is generated by the SMTP receiver.
It MUST appear above any other Received-SPF headers in the
message. The header has the format:
header = "Received-SPF:" [CFWS] result [CFWS]
[ key-value-list ]
result = "Pass" / "Fail" / "TempError" / "SoftFail" /
"Neutral" / "None" / "PermError"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) [";"]
key-value-pair = name [CFWS] "=" ( dot-atom / quoted-string )
dot-atom = ; unquoted word as per RFC2822
quoted-string = ; quoted string as per RFC2822
CFWS = ; comment or folding white space as per RFC2822
The <comment-string> should convey supporting
information for the result, such as <ip>,
<sender> and <domain>.
SPF clients may append zero or more of the following key-value-pairs
at their discretion:
the host name of the SPF client the IP address of the SMTP client the envelope sender address the host name given in the HELO
or EHLO command
the mechanism that matched (if
no mechanisms matched, substitute the word "default".)
if an error was returned,
details about the error
Other key-value pairs may be defined by SPF clients. Until a new key
name becomes widely accepted, new key names should start with "x-".
SPF clients MUST make sure that the Received-SPF header does
not contain invalid characters, is excessively long, or
contain malicious data that has been provided by the sender.
Examples of various header styles that could be generated:
Received-SPF: Pass (mybox.example.org: domain of
myname@example.com designates 192.0.2.1 as permitted sender)
receiver=mybox.example.org; client-ip=192.0.2.1;
envelope-from=<myname@example.com>; helo=foo.example.com;
Received-SPF: Fail (mybox.example.org: domain of
myname@example.com does not designate
192.0.2.1 as permitted sender)
receiver=mybox.example.org;
client-ip=192.0.2.1;
envelope-from=<myname@example.com>;
helo=foo.example.com;
Received-SPF: SoftFail (mybox.example.org: domain of
transitioning myname@example.com discourages
use of 192.0.2.1 as permitted sender)
Received-SPF: Neutral (mybox.example.org: 192.0.2.1 is neither
permitted nor denied by domain of
myname@example.com)
Received-SPF: None (mybox.example.org: myname@example.com does
not designate permitted sender hosts)
Received-SPF: PermError (mybox.example.org: domain
of myname@example.com used an invalid
SPF mechanism)
Received-SPF: TempError (mybox.example.org: error in processing
during lookup of myname@example.com: DNS
timeout)
Many mechanisms and modifiers perform macro interpolation on
part of the term.
domain-spec = macro-string domain-end
domain-end = ( "." toplabel ) / macro-expand
toplabel = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
; LDH rule (See RFC3696)
alphanum = ALPHA / DIGIT
macro-string = *( macro-expand / macro-literal )
explain-string = *( macro-string / SP )
macro-expand = ( "%{" macro-letter transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t"
transformer = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
A literal "%" is expressed by "%%". "%_" expands to a single " " space."%-" expands to a URL-encoded space, viz. "%20". The following macro letters are expanded in term arguments: = <sender>= local-part of <sender>= domain of <sender>= <domain>= <ip>= the validated domain name of <ip>= the string "in-addr" if <ip>
is ipv4, or "ip6" if <ip> is ipv6
= HELO/EHLO domain The following macro letters are only allowed in "exp" text: = SMTP client IP (easily readable format)= domain name of host performing the check= current timestamp
A '%' character not followed by a '{', '%', '-', or '_'
character is a syntax error. So,
-exists:%(ir).sbl.spamhaus.org
is incorrect and will cause check_host() to return a
"PermError". Instead, say
-exists:%{ir}.sbl.spamhaus.org Optional transformers are:
: zero or more digits: reverse value, splitting on dots
by default
If transformers or delimiters are provided, the replacement
value for a macro letter is split into parts. After
performing any reversal operation and/or removal of
left-hand parts, the parts are rejoined using "." and not
the original splitting characters.
By default, strings are split on "." (dots). Note that no
special treatment is given to leading, trailing or
consecutive delimiters, and so the list of parts may contain
empty strings. Macros may specify delimiter characters which
are used instead of ".".
The 'r' transformer indicates a reversal operation: if the
client IP address were 192.0.2.1, the macro %{i} would
expand to "192.0.2.1" and the macro %{ir} would expand to
"1.2.0.192".
The DIGIT transformer indicates the number of right-hand
parts to use, after optional reversal. If a DIGIT is
specified, the value MUST be nonzero. If no DIGITs are
specified, or if the value specifies more parts than are
available, all the available parts are used. If the DIGIT
was 5, and only 3 parts were available, the macro
interpreter would pretend the DIGIT was 3. Implementations
MUST support at least a value of 128, as that is the maximum
number of labels in a domain name.
The "s" macro expands to the <sender> argument. It is
an e-mail address with a localpart, an "@" character, and a
domain. The "l" macro expands to just the localpart. The "o"
macro expands to just the domain part. Note that these
values remain the same during recursive and chained
evaluations due to "include" and/or "redirect". Note also
that if the original <sender> had no localpart, the
localpart was set to "postmaster" in initial processing (see
).
For IPv4 addresses, both the "i" and "c" macros expand to the
standard dotted-quad format.
For IPv6 addresses, the "i" macro expands to a dot-format
address; it is intended for use in %{ir}. The "c" macro may
expand to any of the hexadecimal colon-format addresses
specified in section 2.2. It is
intended for humans to read.
The "p" macro expands to the validated domain name of
<ip>. The procedure for finding the validated domain
name is defined in . If the
<domain> is present in the list of validated domains,
it SHOULD be used. Otherwise, if a subdomain of the
<domain> is present, it SHOULD be used. Otherwise,
any name from the list may be used. If there are no
validated domain name or if a DNS error occurs, the string
"unknown" is used.
The "r" macro expands to the name of the receiving MTA. This
SHOULD be a fully qualified domain name, but if one does not
exist (as when the checking is done by a script) or if
policy restrictions dictate otherwise, the word "unknown"
should be substituted. The domain name may be different than
the name found in the MX record that the client MTA used to
locate the receiving MTA.
The "t" macro expands to the decimal representation of the
approximate number of seconds since the Epoch (Midnight,
January 1st, 1970, UTC). This is the same value as returned
by the POSIX time() function in most standards compliant
libraries.
When the result of macro expansion is used in a domain name
query, if the expanded domain name exceeds 253 characters
(the maximum length of a domain name), the left side is
truncated to fit, by removing successive subdomains until
the total length does not exceed 253 characters.
Uppercased macros expand exactly as their lower case
equivalents, and are then URL escaped. URL escaping for the
non-uric characters is described in .
Note: Domains should avoid using the "s", "l", "o" or "h"
macros in conjunction with any mechanism directive. While
these macros are powerful and allow per-user records to be
published, they severely limit the ability of
implementations to cache results of check_host() and they
reduce the effectiveness of DNS caches.
Implementations should be aware that if no directive
processed during the evaluation of check_host() contains an
"s", "l", "o" or "h" macro, then the results of the
evaluation can be cached on the basis of <domain> and
<ip> alone for as long as the shortest TTL of all the
DNS records involved.
The <sender> is strong-bad@email.example.com.The IPv4 SMTP client IP is 192.0.2.3.The IPv6 SMTP client IP is 5f05:2000:80ad:5800::1.The PTR domain name of the client IP is mx.example.org.
macro expansion
------- ----------------------------
%{s} strong-bad@email.example.com
%{o} email.example.com
%{d} email.example.com
%{d4} email.example.com
%{d3} email.example.com
%{d2} example.com
%{d1} com
%{dr} com.example.email
%{d2r} example.email
%{l} strong-bad
%{l-} strong.bad
%{lr} strong-bad
%{lr-} bad.strong
%{l1r-} strong
macro-string expansion
--------------------------------------------------------------------
%{ir}.%{v}._spf.%{d2} 3.2.0.192.in-addr._spf.example.com
%{lr-}.lp._spf.%{d2} bad.strong.lp._spf.example.com
%{lr-}.lp.%{ir}.%{v}._spf.%{d2}
bad.strong.lp.3.2.0.192.in-addr._spf.example.com
%{ir}.%{v}.%{l1r-}.lp._spf.%{d2}
3.2.0.192.in-addr.strong.lp._spf.example.com
%{d2}.trusted-domains.example.net
example.com.trusted-domains.example.net
IPv6:
%{ir}.%{v}._spf.%{d2} 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.
5.d.a.0.8.0.0.0.2.5.0.f.5.ip6._spf.example.com
This section outlines the major implications that adoption of this
document will have on various entities involved in Internet e-mail.
It is intended to make clear to the reader where this document
knowingly affects the operation of such entities. This section is
not a "how-to" manual, nor a "best practices" document, and is not a
comprehensive list of what such entities should do in light of this
document.
This section is non-normative.
Domains that wish to be compliant with this specification
will need to determine the list of hosts that they allow to
use their domain name in the "HELO" and "MAIL FROM"
identities. It is recognized that forming such a list is
not just a simple technical exercise, but involves policy
decisions with both technical and administrative
considerations.
It can be helpful to publish records that include a
"tracking exists:" mechanism. By looking at the name server
logs, an incompletely list may be generated. For example:
v=spf1 exists:CL.%{i}.FR.%{s}.HE.%{h}._spf.%{d} ?all"
Mailing lists must be aware of how they re-inject mail that
is sent to the list. Mailing lists MUST comply with the
requirement in Section 3.10 and
Section 5.3.6 that say that the
reverse-path MUST be changed to be the address of a person
or other entity who administers the list. While the reasons
for changing the reverse-path are many and long standing,
SPF adds enforcement to this requirement.
In practice, almost all mailing list software in use already
complies with this requirement. Mailing lists that do not
comply, may or may not encounter problems depending on how
access to the list is restricted. Such lists that are
entirely internal to a domain (only people in the domain can
send to or receive from the list) are not affected.
Forwarding services take mail that is received at a mailbox
and direct it to some external mailbox. At the time of this
writing, the near-universal practice of such services is to
use the original reverse-path of a message when re-injecting
it for delivery to the external mailbox. and describe this
action as an "alias" rather than a "mail list". This means
the external mailbox's MTA sees all such mail in a
connection from a host of the forwarding service, and so the
"MAIL FROM" identity will not in general pass authorization.
There are several possible ways that this authorization
failure can be ameliorated. If the owner of the external
mailbox wishes to trust the forwarding service, they can
direct the external mailbox's MTA to skip such tests when
the client host belongs to the forwarding service. Tests
against some other identity may also be used to override the
test against the "MAIL FROM" identity.
For larger domains, it may not be possible to have a
complete or accurate list of forwarding services used by the
owners of the domain's mailboxes. In such cases, white
lists of generally recognized forwarding services could be
employed.
Forwarding services can also solve the problem by using
reverse-paths that contain their own domain. This means
that mail bounced from the external mailbox will have to be
re-bounced by the forwarding service. Various schemes to do
this exist though they vary widely in complexity and
resource requirements on the part of the forwarding service.
Several polular MTAs can change "alias" semantics to
"mailing list" semantics by including an adding another
alias with "owner-" added to the beginning of the alias
name. (e.g. an alias of "friends: george@example.com,
fred@example.org" would need another alias of the form
"owner-friends: localowner")
Entities that offer mail services to other domains such as
sending of bulk mail will may have to alter their mail in
light of the authorization check in this document. If the
reverse-path used for such e-mail uses the domain of the
mail service provider, then the provider needs only to
ensure that their sending host is authorized by their own
SPF record, if any.
If the reverse-path does not use the mail service provider's
domain, then extra care must be taken. The SPF record
format has several options for authorizing the sending MTAs
of another domain (the service provider's)
The authorization check generally precludes the use of arbitrary MTA
relays between sender and receiver of an e-mail message.
Within an organization, MTA relays can be effectively deployed.
However, for purposes of this document, such relays are effectively
invisible. The "MAIL FROM" identity authorization check is a check
between border MTAs.
For mail senders, this means that published SPF records must
authorize any MTAs that actually send across the Internet. Usually,
these are just the border MTAs as internal MTAs simply forward mail
to these MTAs for delivery.
Mail receivers will generally want to perform the authorization check
at the border MTAs. This allows mail that fails to be rejected
during the SMTP session rather than bounced. Internal MTAs then do
not perform the authorization test. To perform the authorization
test other than at the border, the host that first transferred the
message to the organization must be determined, which can be
difficult to extract from headers. Testing other than at the border
is not recommended.
The "MAIL FROM" and "HELO" identity authorizations must not be
construed to provide more assurance than it does. It is
entirely possible for a malicious sender to inject a message
using their own domain in the identities used by SPF, to have
that domain's SPF record authorize the sending host, and yet
the message content can easily claim other identities in the
headers. Unless the user, or the MUA takes care to note that
the authorized identity does not match the other, more
commonly presented identities (such as the From: header), the
user may be lulled into a false sense of security.
There are two aspects of this protocol that malicious parties could
exploit to undermine the validity of the check_host() function:
The evaluation of check_host() relies heavily on DNS. A
malicious attacker could attack the DNS infrastructure and
cause check_host() to see spoofed DNS data, and then
return incorrect results. This could include returning
"Pass" for an <ip> value where the actual domain's
record would evaluate to "Fail". See for a description of the DNS
weaknesses.
The client IP address, <ip>, is assumed to be
correct. A malicious attacker could spoof TCP sequences
to make mail appear to come from a permitted host for a
domain that the attacker is impersonating.
As with most aspects of mail, there are a number of ways
that malicious parties could use the protocol as an avenue
of a distributed Denial-of-Service attack. The processing
limits outlined in are designed to
prevent attacks such as:
Malicious parties could create SPF records that make many
references to the target's domain and then send large
volume mail to other SPF clients that use this SPF record.
These legitimate machines would then present a DNS load on
the target as they fetched the relevant DNS references.
While implementations of check_host() need to limit the
number of DNS lookups, malicious domains could publish
records that exercise or exceed these limits in an attempt
to waste computation effort at their targets when they
send them mail. Malicous domains could also design SPF
records that cause excessive memory or CPU usage.
Malicious parties could send large volume mail
purporting to come from the intended target to a wide
variety of legitimate mail hosts. These legitimate
machines would then present a DNS load on the target as
they fetched the relevant records.
When the authorization check fails, an explanation string may
be included in the reject response. Both the sender and the
rejecting receiver need to be aware that the explanation was
determined by the publisher of the SPF record checked, and is
in general not the receiver. The explanation may contain URLs
that may be malicious, and/or offensive or misleading text.
This is probably less of a concern than it may initially seem
since such messages are returned to the sender, and their
source is the SPF record published by the domain in the
identity claimed by that very sender. To put it another way,
the only people who see malicious explanation strings are
people whose messages claim to be from domains that publish
such strings in their SPF records.
SPF uses information supplied by third parties, such as the
HELO domain name, the return-path and SPF records. This
information is then sent to the receiver in the Received-SPF:
mail headers and possibly returned to the client MTA in the
form of an SMTP rejection message. This information must be
checked for invalid characters and excessively long lines.
The IANA needs to assign a new Resource Record Type and Qtype
from the DNS Parameters Registry for the SPF RR type.
This document is largely based on the work of Meng Weng Wong
and Mark Lentczner. They are not listed as authors by their
request. While, as this section acknowledges, many people
have contributed to this document, a very large portion of the
writing and editing are due to Meng and Mark.
This design owes a debt of parentage to by
Hadmut Danisch and to by Gordon Fecyk.
The idea of using a DNS record to check the legitimacy of an
e-mail address traces its ancestry farther back through
messages on the namedroppers mailing list by Paul Vixie (based on suggestion by Jim Miller) and by
David Green .
Philip Gladstone contributed macros to the specification,
multiplying the expressiveness of the language and making
per-user and per-IP lookups possible.
The authors would also like to thank the literally hundreds of
individuals who have participated in the development of this
design. There are far too numerous to name, but they include:
The folks on the SPAM-L mailing list.The folks on the ASRG mailing list.The folks on the spf-discuss mailing list.The folks on #perl.
The folks in the MARID working group and on the MXCOMP
mailing list.
&rfc1035;
&rfc1123;
&rfc2119;
&rfc2181;
&rfc2234;
&rfc2396;
&rfc2821;
&rfc2822;
&rfc3513;
&rfc1034;
&rfc1983;
&rfc2162;
&rfc3696;
&rfc3833;
The RMX DNS RR Type for light weight sender authenticationWork In ProgressDesignated Mailers ProtocolWork In ProgressRepudiating MAIL FROMDomain-Authorized SMTP Mail
This section is normative and any discrepancies with the ABNF
fragments in the preceding text are to be resolved in favor of this
grammar.
See for ABNF notation. Please note
that as per this ABNF definition, literal text strings (those
in quotes) are case-insensitive. Hence, "mx" matches "mx",
"MX", "mX" and "Mx".
record = version terms *SP
version = "v=spf1"
terms = *( 1*SP ( directive / modifier ) )
directive = [ prefix ] mechanism
prefix = "+" / "-" / "?" / "~"
mechanism = ( all / include
/ A / MX / PTR / IP4 / IP6 / exists )
all = "all"
include = "include" ":" domain-spec
A = "a" [ ":" domain-spec ] [ dual-cidr-length ]
MX = "mx" [ ":" domain-spec ] [ dual-cidr-length ]
PTR = "ptr" [ ":" domain-spec ]
IP4 = "ip4" ":" ip4-network [ ip4-cidr-length ]
IP6 = "ip6" ":" ip6-network [ ip6-cidr-length ]
exists = "exists" ":" domain-spec
modifier = redirect / explanation / unknown-modifier
redirect = "redirect" "=" domain-spec
explanation = "exp" "=" domain-spec
unknown-modifier = name "=" macro-string
ip4-cidr-length = "/" 1*DIGIT
ip6-cidr-length = "/" 1*DIGIT
dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
ip4-network = as per conventional dotted quad notation,
e.g. 192.0.2.0
ip6-network = as per [RFC 3513], section 2.2,
e.g. 2001:DB8::CD30
domain-spec = macro-string domain-end
domain-end = ( "." toplabel ) / macro-expand
toplabel = ALPHA / ALPHA *[ alphanum / "-" ] alphanum
; LDH rule (See RFC3696)
alphanum = ALPHA / DIGIT
macro-string = *( macro-expand / macro-literal )
explain-string = *( macro-string / SP )
macro-expand = ( "%{" macro-letter transformer *delimiter "}" )
/ "%%" / "%_" / "%-"
macro-literal = %x21-24 / %x26-7E
; visible characters except "%"
macro-letter = "s" / "l" / "o" / "d" / "i" / "p" / "h" /
"c" / "r" / "t"
transformer = *DIGIT [ "r" ]
delimiter = "." / "-" / "+" / "," / "/" / "_" / "="
name = ALPHA *( ALPHA / DIGIT / "-" / "_" / "." )
header = "Received-SPF:" [CFWS] result [CFWS]
[ key-value-list ]
result = "Pass" / "Fail" / "TempError" / "SoftFail" /
"Neutral" / "None" / "PermError"
key-value-list = key-value-pair *( ";" [CFWS] key-value-pair ) [";"]
key-value-pair = name [CFWS] "=" ( dot-atom / quoted-string )
dot-atom = ; unquoted word as per RFC2822
quoted-string = ; quoted string as per RFC2822
CFWS = ; comment or folding white space as per RFC2822
These examples are based on the following DNS setup:
; A domain with two mail servers, two hosts
; and two servers at the domain name
$ORIGIN example.com.
@ MX 10 mail-a
MX 20 mail-b
A 192.0.2.10
A 192.0.2.11
amy A 192.0.2.65
bob A 192.0.2.66
mail-a A 192.0.2.129
mail-b A 192.0.2.130
www CNAME example.com.
; A related domain
$ORIGIN example.org
@ MX 10 mail-c
mail-c A 192.0.2.140
; The reverse IP for those addresses
$ORIGIN 2.0.192.in-addr.arpa.
10 PTR example.com.
11 PTR example.com.
65 PTR amy.example.com.
66 PTR bob.example.com.
129 PTR mail-a.example.com.
130 PTR mail-b.example.com.
140 PTR mail-c.example.org.
; A rogue reverse IP domain that claims to be
; something it's not
$ORIGIN 0.0.10.in-addr.arpa.
4 PTR bob.example.com.
These examples show various possible published records for
example.com and which values if <ip> would cause
check_host() to return "Pass". Note that <domain> is
"example.com".
v=spf1 +all -- any <ip> passes v=spf1 a -all -- hosts 192.0.2.10 and 192.0.2.11 pass v=spf1 a:example.org -all -- no sending hosts pass since example.org has no A records v=spf1 mx -all -- sending hosts 192.0.2.129 and 192.0.2.130 pass v=spf1 mx:example.org -all -- sending host 192.0.2.140 passes v=spf1 mx mx:example.org -all -- sending hosts 192.0.2.129, 192.0.2.130, and
192.0.2.140 pass
v=spf1 mx/30 mx:example.org/30 -all -- any sending host in 192.0.2.128/30 or
192.0.2.140/30 passes
v=spf1 ptr -all -- sending host 192.0.2.65 passes (reverse IP is valid
and in example.com)
-- sending host 192.0.2.140 fails (reverse IP is valid, but not
in example.com)
-- sending host 10.0.0.4 fails (reverse IP is not valid) v=spf1 ip4:192.0.2.128/28 -all -- sending host 192.0.2.65 fails-- sending host 192.0.2.129 passes These examples show the effect of related records:
example.org: "v=spf1 include:example.com
include:example.net -all"
This record would be used if mail from example.org actually came
through servers at example.com and example.net. Example.org's
designated servers are the union of example.com and example.net's
designated servers.
la.example.org: "v=spf1 redirect=example.org"ny.example.org: "v=spf1 redirect=example.org"sf.example.org: "v=spf1 redirect=example.org"
These records allow a set of domains that all use the same mail
system to make use of that mail system's record. In this way, only the
mail system's record needs to updated when the mail setup changes.
These domains' records never have to change.
Imagine that, in addition to the domain records listed above, there
are these:
$Origin _spf.example.com.
mary.mobile-users A 127.0.0.2
fred.mobile-users A 127.0.0.2
15.15.168.192.joel.remote-users A 127.0.0.2
16.15.168.192.joel.remote-users A 127.0.0.2
The following records describe users at example.com who mail from
arbitrary servers, or who mail from personal servers.
example.com:
v=spf1 mx
include:mobile-users._spf.%{d}
include:remote-users._spf.%{d}
-all
mobile-users._spf.example.com:
v=spf1 exists:%{l1r+}.%{d}
remote-users._spf.example.com:
v=spf1 exists:%{ir}.%{l1r+}.%{d}